MTA Chairman Joseph Lhota yesterday took the first step toward a run for City Hall, resigning from his post at the transit agency to test the waters in the crowded 2013 mayoral race.

His last day at the helm is New Year’s Eve.

Lhota — a former top deputy mayor under Rudy Giuliani — said he’ll make a final decision early next month.

“This will be a life-defining decision and one that I will be seriously considering in the upcoming weeks,” he said at a press conference after an MTA board meeting.

His entry into the race — which has been lacking a strong Republican candidate — would be welcome news to the city’s business community.

“I just think a strong GOP candidate is important to bring balance to the race,” said Kathy Wylde, the leader of the business group Partnership for New York City.

“If he enters the race, I think he has a great story to tell about what it took to transform New York into a safe, manageable city.”

Because of his position with the MTA, Lhota under state law had been barred from planning a run for public office.

But with this decision, on New Year’s Day, he can begin talking to potential donors.

Sources told The Post that many in Giuliani’s circle are confident that Lhota can raise $10 million.

One person whose support Lhota won’t be receiving is Mayor Bloomberg.

According to a City Hall source, Bloomberg dislikes him, especially after Lhota said the mayor acted “like an idiot” when Hizzoner speculated on the reopening of the Queens Midtown Tunnel after Hurricane Sandy.

Lhota’s entry likely hurts the mayoral ambitions of City Council Speaker Christine Quinn, an all-but-announced candidate who had been positioning herself as the most centrist in the Democratic field.

The 2013 field also could include city Comptroller John Liu, Public Advocate Bill de Blasio and former Comptroller Bill Thompson.

A spokesman for former Bronx Borough President Adolfo Carrión — who is mulling a mayoral bid as a Republican — said he still plans to run.